FIG. 1 shows a schematic view of a conventional suction device in use. As shown in FIG. 1, suction device 100 includes a suction disc 10, a shell 11 and a wrench unit 12. Shell 11 further includes a connecting element 111 for connecting a heavy object 15 of any kind. In this manner, heavy object 15 can be hung on a vertical surface 90 with a suction device 100. Suction device 100 uses suction disc 10 to attach to vertical surface 90. A compressible space 101 is formed between suction disc 10 and vertical surface 90. Wrench element 12 is for pulling the center of suction disc 10 away from vertical surface 90 so that compressible space 101 is enlarged to create a suction similar to the vacuum suction. In addition, when wrench element 12 moves, wrench element 12 also applies force to shell 11 tightly against the surface on the circumference of suction disc 10 to prevent air from leaking into compressible space 101. In other words, the more tightly shell 11 is pressed against suction disc 10, the more durable the suction will be.
However, in practical use, suction device 100 may suffer other external forces unaccounted for. Referring to FIG. 1, if heavy object 15 is hung on connecting element 111 of shell 11 when suction device 100 is attached to vertical surface 90, lower edge 112 of shell 11 exerts the maximum force on the right side surface wall of suction disc 10 due to the hanging arm mechanism, while upper edge 113 of shell 11 is barely in contact with suction disc 10. The distribution of the forces will affect the suction of suction disc 10. If suction disc 10 is divided into two parts with a middle horizontal line, the upper part of suction disc 10 will has a smaller contact area with vertical surface 90 than the lower part of suction disc 10. Therefore, compressible space 101 will have a cross-section of a drop shape. In this manner, after a long time pull, the contact area of the upper part of suction disc will become less and less. As lower edge 113 of shell 11 does not exert force on the side wall of the upper part of suction disc 10, the air will eventually leak into compressible space 101 through the upper part of suction disc 10, which leads to the loss of suction of suction disc 10 and disengagement from vertical surface 90. It is imperative to devise a structure for suction device to enhance the durability of the suction.